1. Field of the Invention
The present invention relates to a power converter, and more specifically relates to a control circuit of the power converter.
2. Description of Related Art
Power converters have been widely used to provide regulated outputs. For safety, the power converter must provide galvanic isolation between its primary side and secondary side. A transformer is usually equipped to provide the isolation. FIG. 1 shows a traditional power converter having a transformer 10. The energy is stored into the transformer 10 during a switch 20 is on, in which the switch 20 can be power transistor or a power MOSFET. The energy will be discharged to the output terminal of the power converter when the switch 20 is switched off. A control circuit 25 generates a switching signal VG at an output terminal OUT of the control circuit 25 to control the on/off of the switch 20 and regulate the output of the power converter. The switch 20 is turned on when the switch signal VG is enabled.
A current-sense device 24 is connected between the switch 20 and the ground to sense a switching current IP of the transformer 10 and generate a current-sense signal VCS for the switching control. The current-sense device 24 can be a current-sense resistor. The current-sense device 24 is further coupled to a current-sense terminal CS of the control circuit 25 to transmit the current-sense signal VCS to the current-sense terminal CS. A ground terminal GND and a compensation terminal COM of the controller 25 are coupled to the ground and a compensation capacitor 26 respectively.
The transformer 10 includes a primary winding NP, a secondary winding NS and an auxiliary winding NAUX. The primary winding NP is coupled from an input voltage VIN to the switch 20. The secondary winding NS connects a rectifier 15. A filter capacitor 17 is coupled to the rectifier 15 and the secondary winding NS. Once the switch 20 is turned off, the auxiliary winding NAUX will produce a reflected voltage VF correlated to the output voltage VO. The reflected voltage VF can be utilized to detect the discharge time of the transformer 10 and/or feedback the output voltage VO. A reflected voltage control technique has been disclosed in U.S. Pat. No. 4,302,803 “Rectifier-Converter Power Supply with Multi-Channel Flyback Inverter”, issued to Randolph D. W. Shelly. However, foregoing prior art cannot measure an accuracy reflected voltage VF from the transformer 10, particularly in the light load conditions. FIG. 2 shows the voltage waveforms of the power converter in light load condition. The discharge time TDS of the transformer 10 can be expressed as,
                              T          DS                =                              (                                          V                IN                                                              V                  O                                +                                  V                  D                                                      )                    ×                                    W              NS                                      W              NP                                ×                      T            ON                                              (        1        )            where VIN is the input voltage of the power converter; WNP and WNS are respectively the winding turns of the primary winding NP and the secondary winding NS of the transformer 10; VD is a forward voltage drop of the rectifier 15; TON is an on time of the switching signal VG.
The reflected voltage VF is connected to the control circuit 25 through resistors 21 and 22 to produce a voltage-sense signal VDET for the reflected voltage VF detection. Since the on time TON and the discharge time TDS are short in the light load conditions, a parasitic capacitor 23 and resistors 21, 22 forms a low pass filter that will distort the waveform of the voltage-sense signal VDET. Normally a lower reflected voltage VF is thus detected. This drawback is the main object of the present invention to overcome.